It doesn’t matter what happened in that bathroom. You don’t repeatedly kick someone in the head who is laying on the ground not threatening you. You may throw a punch or demand that the police be called and try to keep them there to answer for whatever it is that “may” have happened in the bathroom but you don’t join into the gang of people kicking them in the head while they are laying on the ground. If they are laying on the ground, your life is no longer threatened and you get the hell out and call the police. There is ZERO excuse for what happened to this human being.
That is really all that needs to be said. You don't continue to hit someone who is not hitting back. And you don't leave the immediate vicinity, only to return moments later and attempt to kidnap the victim. Like you said, ZERO excuse.